Assessment of parameters describing representativeness of air quality in-situ measurement sites

Abstract

The atmospheric layer closest to the ground is strongly influenced by variable surfacefluxes (emissions, surface deposition) and can therefore be very heterogeneous. Inorder to perform air quality measurements that are representative of a larger domainor a certain degree of pollution, observatories are placed away from population cen-5tres or within areas of specific population density. Sites are often categorised basedon subjective criteria that are not uniformly applied within different administrative do-mains. A novel approach for the assessment of parameters reflecting site represen-tativeness is presented here, taking emissions, deposition and transport towards 34sites covering Western and Central Europe into account. These parameters are di-10rectly inter-comparable among the sites and can be used to select sites that are, onaverage, more or less suitable for data assimilation and comparison with satellite andmodel data. Advection towards these sites was simulated by backward LagrangianParticle Dispersion Modelling (LPDM) to determine the sites’ annual catchment areasfor the year 2005 and advection times of 12, 24 and 48 h. Only variations caused by15emissions and transport during these periods were considered assuming that thesedominate the short-term variability of most but especially short lived trace gases. Theparameters of representativeness derived were compared between sites and a novel,uniform and observation-independent categorisation of the sites based on a clusteringapproach was established. Six groups of European background sites were identified20ranging from very remote coastal to polluted rural sites. These six categories explained50 to 80% of the inter-site variability of median mixing ratios and their standard devia-tion for NO2and O3, while differences between group means of the longer lived tracegas CO were insignificant. The derived annual catchment areas strongly dependedon the applied LPDM and input wind fields, the catchment settings and the year of25analysis. Nevertheless, the parameters of representativeness showed considerablyless variability than the catchment geometry, supporting the robustness of the derivedstation categorisation.